1. Field of the Invention
This invention relates to the recovery of oil from a subterranean formation. More specifically, this invention relates to an enhanced oil recovery process which utilizes surfactants.
2. Description of the Prior Art
The petroleum industry has recognized for many years that only a small fraction of the original oil in place in a reservoir is expelled by natural mechanisms. It is also well-known that conventional methods of supplementing natural recovery are relatively inefficient. Typically, a reservoir may retain half its original oil even after the application of currently available methods of secondary recovery. Accordingly, there is a continuing need for improved recovery methods which will substantially increase the ultimate yield of petroleum from subterranean reservoirs.
Waterflooding is by far the most widely practiced method for recovering oil from a formation after naturally occurring forces in the formation have declined in their ability to expel oil. In waterflooding, water is typically injected through an input well to drive oil to offset producing wells. Much of the current work in oil recovery technology has been directed toward improving the efficiency of waterflooding processes or developing alternative processes.
Surface-active agents or surfactants are one class of materials which have been proposed for improving the efficiency of waterflooding processes. Much of the oil that is retained in the reservoir after a typical waterflood is in the form of discontinuous globules or discrete droplets which are trapped within the pore spaces of the reservoir. Because the normal interfacial tension between the reservoir oil and water is so high, these discrete droplets are unable to sufficiently deform to pass through narrow constrictions in the pore channels. When surface-active agents are added to the flood water, they lower the interfacial tension between the water and the reservoir oil and permit the oil droplets to deform and flow with the flood water. It is generally conceded that the interfacial tension between the flood water and the reservoir oil must be reduced to less than 0.1 dyne/cm for additional recovery.
More recently, surfactants have been used advantageously in flooding with microemulsions or micellar dispersions. Microemulsions are stable, transparent or translucent mixtures of a liquid hydrocarbon, water and a surfactant. Optionally, a co-solvent (such as alcohol), cosurfactants, and electrolytes may be present in the mixture. A discussion of the properties of microemulsions may be found in the following references: R. N. Healy and R. L. Reed, "Multiphase Microemulsion Systems", Society of Petroleum Engineers Journal, pp. 147-160 (1976); and P. A. Winsor, "Solvent Properties of Amphiphilic Compounds", Butterworth's Scientific Publications, London (1954).
In practice, a microemulsion slug is typically injected into the formation, followed by an aqueous slug thickened with a polymer to provide a mobility buffer for improved sweep efficiency. The microemulsion and thickened aqueous slug are then displaced through the formation by flooding water to drive crude oil through the formation to one or more production wells for recovery.
Although flooding processes utilizing surfactants may be effective in obtaining additional oil from subterranean oil reservoirs, a number of shortcomings continue to detract from their value. One difficulty which has been observed in the use of surfactants in general is the tendency of the surfactants to be depleted from the injected solution. It has been demonstrated that at least a portion of the surface-active agents is adsorbed on the rock surface of the reservoir and a portion is physically entrapped within the pore spaces of the rock matrix. It is also known that many surfactants react with ionic substances in the water within the formation and are thereafter unable to interact at the oil/water interface to reduce the interfacial tension. The surfactant depletion can seriously reduce oil recovery efficiency.
A promising approach for reducing the amount of surfactant retained by the formation has been to use sacrificial compounds, usually in a preflush solution injected into the formation before injection of the surfactant-containing solution. The compounds are "sacrificial" in the sense that they become trapped in the formation thereby reducing retention of the more expensive surfactant contained in the trailing surfactant solution.
Lignosulfonates form one class of compounds which have been found to have excellent properties as sacrificial agents. They are economically attractive since being by-products of the pulping industry, they are plentiful and cost less than surfactants used in enhanced oil recovery methods. The use of various lignosulfonates has been disclosed extensively in the patent literature: U.S. Pat. Nos. 3,384,171; 4,133,385; 4,142,582; 4,157,115; 4,172,497; 4,172,498; 4,196,777; 4,252,194; and 4,271,906. While some of the earlier patents require that the formation be contacted with the lignosulfonates before injecting surfactant solution, later patents have suggested that it is preferable to inject lignosulfonates in admixture with the surfactant solution.
The '194 patent discloses a use for lignosulfonate other than as a sacrificial agent. It teaches a method for controlling the mobility of fluids during the injection process by using thickened lignosulfonates as a mobility buffer. Heating the lignosulfonates in excess of 200.degree. F. for a sufficient period of time causes its viscosity to increase. It can then be used as the polymer flood slug during a micellar flooding process and as the polymer solution during a polymer flood or waterflood, but it does not function as a sacrificial agent.
The '906 patent involves using lignosulfonates as a sacrifical agent injected into the formation in the form of a plurality of separate, discrete slugs and alternating these injections with injections of similarly small slugs of surfactant solution. After all the slugs are injected, a viscous mobility buffer followed by drive water, is used to displace the fluids. This method does not disclose the use of lignosulfonates as an afterflush but is actually a more complicated variation of the admixture method.
While the use of lignosulfonates as preflushes or admixtures has, to some extent, reduced surfactant retention by a formation, other problems have arisen. For example, when lignosulfonates are used in a preflush solution, the length of time before any additional amount of oil is recovered will be increased since lignosulfonates, by themselves, will not recover oil. This can reduce the overall economics of the process. In addition, preflushes in general are often not as effective as expected because of the tremendous reservoir volumes. There is no assurance that the flow path of the surfactant solution will sufficiently coincide with that of the preflush.
In part, injecting lignosulfonates in admixture with surfactant mitigates these problems. However, yet other problems arise due to such admixture, especially where microemulsions are being used as the surfactant-containing liquid. These problems relate to chromatographic separation of the surfactant and lignosulfonate resulting in some of the same problems mentioned above. Furthermore, experience has shown that more complex mixing schemes tend to be subject to errors in the field.